1. Field of the Invention
The present invention relates to a gas sensor, which holds a sensor element for measuring a gas to be measured in a metal shell or an outer tube, and has a ventilation structure for ensuring ventilation between the inside and the outside of the outer tube.
2. Description of the Related Art
A gas sensor having a sensor element for detecting the concentration of a specific gas component (e.g., oxygen) in an exhaust gas discharged from an engine using a solid electrolyte body containing a ceramic such as zirconia is well-known in the art. For example, a sensor element of an oxygen sensor for detecting the concentration of oxygen includes a detection electrode exposed to the exhaust gas and a reference electrode exposed to a reference gas (commonly, the atmosphere). The electrodes are formed on an outer or inner surface of the sensor element, and are paired with a counter electrode on opposing sides of a solid electrolyte body. The sensor element detects the concentration of oxygen in the exhaust gas in accordance with the difference in partial pressure of oxygen between two atmospheres spaced apart from the solid electrolyte body, namely between the exhaust gas and the reference gas.
This oxygen sensor includes a metal shell for holding the sensor element, and an outer tube for covering the periphery of the sensor element, and a seal member (grommet) may be assembled thereto to block the outer tube. In addition, the seal member has a lead wire insert hole for inserting a lead wire (a sensor output lead wire and a heater lead wire), and an atmosphere communication hole (or, a cylindrical hole) for introducing the reference gas toward the reference electrode of the sensor element by ensuring ventilation between the inside and the outside of the outer tube. Also, a filter member for introducing the atmosphere into the outer tube (air ventilation) and preventing the entry of water droplets (waterproofing properties) is provided in the atmosphere communication hole of the plug member.
In addition, as disclosed in the Patent Document 1 or 2, a gas sensor having a protection member for covering the atmosphere communication hole of the seal member from a rear side (an outer side) of the outer tube to protect the filter member is already known. Providing the protection member as mentioned above prevents the filter member from being directly exposed to the outside and also prevents the filter member from being damaged due to an impact from the outside, caused by contact with plants or trees or a collision with a flying stone. This protection member has a ventilation portion with a smaller opening than the atmosphere communication hole, and the ventilation portion cover the filter member. Also, the protection member has a covering portion, which is outside the line of sight and communicates with the atmosphere communication hole from the gap with the seal member. The protection member is connected to the outer tube by means of an arm.    Patent Document 1 JP-A-2008-292459    Patent Document 2 JP-A-2008-292460
3. Problems to be Solved by the Invention
However, among the protection members of a gas sensor disclosed in the Patent Documents 1 and 2, the protection member (see FIG. 14 of the Patent Document 2) having a plurality of small openings through the ventilation unit in an axial direction has a problem in that the openings may be clogged since each opening is too small.
Meanwhile, if a protrusion (see FIGS. 3 and 5 of Patent Document 1 and FIG. 12 of Patent Document 2) having an opening and protruding toward a rear end (an outer side) with a dome shape is provided on the ventilation unit, and the opening of the protrusion of the protection member communicates with the atmosphere communication hole by the protection member, then the opening is not easily clogged and good air ventilation can be ensured. However, if the protrusion protrudes from the rear end (the outer side) further than the rear end of the seal member when the protection member having the protrusion is disposed on the seal member to cover the filter member, a lead wire may be hooked by the edge of the protrusion when being installed, leading to a concern that damage to the lead wire may result.